1. Field of the Present Invention
The present invention relates to a display device. More particularly, the present invention relates to a liquid crystal display device of horizontal electric field type which is adapted to increase fabrication yield and to decrease manufacturing cost.
2. Related Arts
A liquid crystal display panel for use in liquid crystal display device includes: a TFT substrate on which pixels, each including a pixel electrode, a thin film transistor (TFT) and the like are arranged in a matrix form; a counter substrate opposed to the TFT substrate and formed with a color filter and the like in corresponding relation to the pixel electrode of the TFT substrate; and liquid crystal sandwiched between the TFT substrate and the counter substrate. An image is formed by controlling transmission of light through liquid crystal molecules on a per pixel basis.
The liquid crystal display device has a flat and lightweight structure and hence, finds more and more applications in various fields. Compact size liquid crystal display devices are widely used in cell phones, Digital Still Cameras (DSCs) and the like. Viewing angle characteristics are important in the liquid crystal display devices. The viewing angle characteristics refer to a phenomenon that the image is varied in luminance or chromaticity depending upon whether it is viewed from front or at oblique angle. An In-Plane Switching (IPS) type device capable of operating the liquid crystal molecules by applying a horizontal electric field exhibits excellent viewing angle characteristics.
There are known various types of IPS liquid crystal display devices. An increased transmittance, for example, can be achieved by a type wherein a common electrode is formed of a flat solid layer and an interdigital pixel electrode is laid on the common electrode with an insulating film interposed therebetween and wherein the liquid crystal molecules are rotated by an electric field induced between the pixel electrode and the common electrode. A similar characteristic is afforded by a system having a converse configuration wherein the pixel electrode is formed in a rectangle shape and the common electrode including slits is overlaid on the pixel electrode with the insulating film interposed therebetween and wherein the liquid crystal molecules are rotated by the electric field induced between the common electrode and the pixel electrode. Of these, the system wherein the pixel electrode is formed in the rectangle shape and the common electrode including the slits is overlaid thereon with the insulating film interposed therebetween is going mainstream because this system permits the reduction of the number of conductive films, insulating films or the like.
JP-A No. 2009-168878 discloses another example of the IPS system having a structure wherein a gate electrode and the common electrode are formed on the same layer and wherein the interdigital pixel electrode is formed with a gate insulating film and a protective insulating film interposed therebetween.
FIG. 15 is a plan view showing a pixel configuration on an IPS TFT substrate, which is a subject of the present invention. FIG. 16 is a sectional view taken on the line C-C in FIG. 15. FIG. 15 and FIG. 16 show the configuration wherein a common electrode 108 including slits 1081 is overlaid on a rectangular pixel electrode 106 with an insulating film interposed therebetween. It is noted that the configuration shown in FIG. 15 is different from the IPS structure disclosed in JP-A No. 2009-168878.
Referring to FIG. 15, the pixel electrode 106 is formed on a region enclosed by a picture signal line 20 and a scan line 10. The TFT controlling the supply of a picture signal to the pixel electrode 106 is formed on the scan line 10. That is, the scan line 10 of FIG. 15 doubles as a gate electrode 101 of the TFT. Formed on the scan line 10 is a semiconductor layer 103, on which a drain electrode 104 and a source electrode 105 are formed. The drain electrode 104 is branched from the picture signal line 20. The source electrode 105 is connected to the pixel electrode 106.
The pixel electrode 106 has a rectangular shape. The common electrode 108 including the slits 1081 is overlaid on the pixel electrode 106 with an inter-layer insulating film 107 (not shown) interposed therebetween. The common electrode 108 is common to individual pixels. In FIG. 15, the common electrode 108 is shown cross-hatched.
FIG. 16 is a sectional view taken on the line C-C in FIG. 15. FIG. 16 also shows a configuration of a counter substrate 200 not shown in FIG. 15. Referring to FIG. 16, the scan line 10 doubling as the gate electrode 101 is formed on a TFT substrate 100, while a gate insulating film 102 is overlaid on the scan line 10. The semiconductor layer 103 is formed over the gate electrode 101 with the gate insulating film 102 interposed therebetween. The drain electrode 104 and the source electrode 105 are laid on the semiconductor layer 103. The source electrode 105 is extended on the gate insulating film 102 to be connected with the pixel electrode 106. As shown in FIG. 15, the pixel electrode 106 has the rectangle shape. In FIG. 16, the picture signal line 20 is formed on the right side of the pixel electrode 106 as spaced a distance therefrom. The picture signal line 20, drain electrode 104 and source electrode 105 are formed at the same time, and followed by the formation of the pixel electrode 106. The drain electrode 104 and the like are formed from Cr, for example, while the pixel electrode 106 is formed from ITO.
The inter-layer insulating film 107 is formed from SiN or the like, covering the pixel electrode 106, picture signal line 20, source electrode 105 and drain electrode 104. The common electrode 108 is formed from ITO and overlaid on the inter-layer insulating film 107. The common electrode 108 is formed as a flat solid film common to the individual pixels. However, the common electrode is formed with the slits 1081, as shown in FIG. 1, at places corresponding to the pixel electrode 106. When a picture signal is applied to the pixel electrode 106, lines of electric force are produced between the pixel electrode 106 and the common electrode 108 as shown in FIG. 16. The lines of electric force cause liquid crystal molecules 301 to rotate while an image is formed by controlling the amount of light transmitted through a liquid crystal layer 300. An alignment film 109 for initial orientation of the liquid crystal molecules 301 is overlaid on the common electrode 108.
Referring to FIG. 16, the liquid crystal layer 300 extends over the TFT substrate 100 and is sandwiched between the TFT substrate 100 and the counter substrate 200. The counter substrate 200 is formed with a color filter 201 at an area corresponding to the pixel electrode 106 on the TFT substrate 100. The counter substrate 200 is further formed with black matrices 202 at places corresponding to the TFT, the picture signal line 202 and the like on the TFT substrate 100. An overcoat film 203 is formed, covering the color filter 201 and the back matrices 202. The overcoat film 203 is provided for preventing reaction of the color filter 201 with the liquid crystal layer 300 and planarizing a contact plane with the liquid crystal layer 300. The alignment film 109 for initial orientation of the liquid crystal molecules 301 is formed on the overcoat film 203.
The IPS device of such a configuration has the following problem. As shown in FIG. 17, a conductive foreign substance 500 inadvertently allowed to enter between the pixel electrode 106 and the common electrode 108 shorts out the pixel electrode 106 and the common electrode 108 so that this pixel is disabled, lapsing into a pixel, defect. Although a very few pixel defects are allowable, a significant number of pixel defects lead to failure of the liquid crystal display device itself. Therefore, the presence of the conductive foreign substance 500 as shown in FIG. 17 leads to a decrease in the fabrication yield of the liquid crystal display device.
However, it is difficult to remove the conductive foreign substance 500 completely from the manufacturing steps. The present invention is directed to an increase in the fabrication yield of the liquid crystal display device by obscuring the pixel defect even if the conductive foreign substance invades the inter-layer insulating film 107.